The present invention relates to an optical characteristic measurement system, and more particularly to that which can measure an optical characteristic of an optical device precisely by eliminating effect of the polarization-dependent loss of the optical path of the measurement system.
As represented by the optical communication system, application of optical energy, having excellent linearity and ease of propagation, to information transmission technology and information processing technology is now studied earnestly, and WDM (Wavelenth-Division-Multiplex) transmission is considered to be a main current of high-capacity communication media in the near future.
The WDM transmission is a technology for transmitting a lot of information by way of a single fiber by superimposing a plurality of carrier waves having different wavelengths modulated with different base-band signals. The superimposed carrier waves can be considered to be a base-band signal whereby a still higher frequency carrier wave is to be modulated and further superimposed into another multi-frequency optical wave.
FIG. 7 is a spectrum chart schematically illustrating a concept of the WDM signal, wherein optical power values are represented by a longitudinal axis and wave lengths are represented by a transversal axis. Around carrier waves 2, 3 and 4 having respective central wavelengths .lambda..sub.1, .lambda..sub.2, and .lambda..sub.3, spectrum bands 5, 6 and 7 are spread occupying bandwidths corresponding to bandwidths of respective base-band signals.
The optical fiber used for optical communication has a far wider bandwidth compared to an electrical cable and can transmit as many carrier waves as the bandwidth permits. Here, it is necessary for correctly reproducing base-band signals that each of the spectrum bands 5, 6 and 7 is not overlapped with each other. However, the spectrum bands are inevitably spread wider than necessary because of bandwidth characteristics of optical devices, such as a modulator/demodulator used in the optical transceivers, and hence, the wide bandwidth of the optical fiber cannot be used fully.
Therefore, performance of an optical communication system can be said to depend greatly on the optical characteristics of the optical devices used in the communication system, and it is very important to precisely evaluate the optical characteristics of the optical devices.
FIG. 8 is a block diagram illustrating a configuration of a conventional optical characteristic measurement system.
The conventional optical characteristic measurement system of FIG. 8 has a multi-wave optical source comprising distributed-feedback laser diodes (hereinafter abbreviated as the DFB-LDs) 10.sub.1 to 10.sub.8 each generating a signal light of a wavelength different from each other, optical switches 11.sub.1 to 11.sub.8 each provided for switching ON/OFF the output light of a respective one of the DFB-LDs 10.sub.1 to 10.sub.8, and an optical coupler 12 functioning as an optical synthesizer for synthesizing the light output from of the optical switches 11.sub.1 to 11.sub.8.
The multi-wave optical source generates a signal light having one or more desired wavelengths different from each other, by synthesizing signal lights selected by the optical switches 11.sub.1 to 11.sub.8 from among the signal lights outputted from the DFB-LDs 10.sub.1 to 10.sub.8 using the optical coupler 12.
The signal light synthesized by the optical coupler 12 is inputted to an optical attenuator 14 after being amplified by an erbium-doped optical-fiber amplifier (EDFA) 13 working as an optical direct amplifier. The optical attenuator 14 is provided for adjusting the light intensity of the signal light, which is performed by controlling the attenuation factor of the optical attenuator 12 with reference to the power value measured by a first optical power meter 16.sub.1 to which is inputted a part of the signal light splitted from a first optical coupler 15 functioning as an optical branch.
The first optical power meter 16.sub.1 measures a power value of the signal light inputted thereto, and the measured value indicates an integral of the power values of each spectrum band when the signal light is a WDM signal such as illustrated in FIG. 7.
The other part of the signal light outputted from the optical attenuator 14 and splitted by the first optical coupler 15 is inputted to a first optical isolator 17, which is provided for suppressing optical noises leaking from the input terminal of an optical component to be connected to the output terminal of the first optical isolator 17, so that the optical noises do not reach the multi-wave optical source. The output light of the first optical isolator 17 is inputted to a first optical switch 18.
The first optical switch 18 switches the optical path of its input light. Here, the first optical switch 18 selects either an optical path connected to a second optical switch 20 passing through a DUT (Device Under Test) 19, that is, an optical device to be measured, or an optical path directly connected to the second optical switch 20.
The signal light arriving at the second optical switch 20 through either one of the two paths is inputted to a second optical isolator 21, which is inserted for suppressing optical noises leaking from an input terminal of a second optical coupler 22 connected to the output terminal of the second optical isolator 21, so that the optical noises do not spread toward the optical source side.
The signal light inputted to the second optical coupler 22, functioning as an optical branch, is split and inputted to an optical spectrum analyzer 23 and to a second optical power meter 16.sub.2, to be measured respectively.
The second optical power meter 16.sub.2 measures a power value of the signal light inputted thereto, and the measured value indicates an integral of the power values of each spectrum band when the signal light is a WDM signal such as illustrated in FIG. 7, in the same way as with the first optical power meter 16.sub.1. The optical spectrum analyzer 23 divides the inputted signal light into frequency components and measures a power value of each of the frequency components.
Since optical devices of the WDM communication system should have sufficient performance in every frequency band used in the WDM communication system, it is very important to evaluate the optical characteristics of the optical devices for each frequency component using the optical spectrum analyzer 23.
In the conventional optical characteristic measurement system having such a configuration as above described, the optical characteristic of the DUT 19 is evaluated according to the difference between the optical powers of the signal light measured passing through the DUT 19 and the signal light not passing through the DUT 19, when the optical characteristics of an optical amplifier is measured as the DUT 19, for example.
However, the optical components in the optical characteristic measurement system have inevitably their own polarization-dependent losses, that is, transmission losses of the optical power varying according to change of polarization direction of the transmission light.
FIG. 9 is a graphic chart schematically illustrating a variation of optical power due to polarization-dependent loss in the measurement system, wherein a measured value 26 of the optical power of a signal light, having a proper power level 25, changes within a variation range 27, according to the polarization variation of the signal light.
The polarization variation may be represented by a function of time. Therefore, the measured value 26 may be said to change within the variation range 27 according to time, which means different power values may be obtained from the same signal light when it is measured at different time points, resulting in a measurement error caused by the variation of the measured value due to the polarization-dependent losses of the optical components in the measurement system.
This measurement error due to the polarization-dependent loss is also inevitable even when the signal light is measured using the optical spectrum analyzer which can measure the optical power for each frequency component.
As described above, optical characteristics of an optical device can not be measured with sufficient precision using the conventional measurement system because of the polarization-dependent loss, and the measurement results obtained without consideration of the polarization-dependent loss are also not been stable.
As to the optical transmission system, there have been proposed some techniques for reducing the effect of polarization-dependent loss by scrambling the polarization direction of the transmission light, an example of which is disclosed in a Japanese patent application laid open as Provisional Publication No. 09-186655.
However, only by reducing the effect of the polarization-dependent loss from the transmission signal, the high-capacity and high-performance WDM communication system of the future cannot be realized. As described above, optical characteristics of each optical device of the WDM communication system should be precisely and stably evaluated for each wavelength, or frequency component. Therefore, it is essential to provide an optical characteristic measurement system which can measure optical characteristics of an optical device for each wavelength, making use of a measurement device such as the optical spectrum analyzer, precisely and stably without being affected by polarization-dependent loss.